1. Field of the Invention
The present invention relates to a perovskite-oxide film, a piezoelectric (ferroelectric) film, a piezoelectric (ferroelectric) device using the perovskite-oxide film, and a liquid discharge device using the piezoelectric device.
2. Description of the Related Art
Perovskite oxides are currently used in piezoelectric devices, switch devices, and the like since perovskite oxides exhibit ferroelectricity. For example, lead titanate zirconate (PZT) is known as a perovskite oxide exhibiting satisfactory piezoelectric characteristics. PZT is a ferroelectric material having spontaneous polarization even when no electric field is applied to PZT, and is reported to exhibit high piezoelectric performance at and near a morphotropic phase boundary (MPB).
However, currently, demands for higher piezoelectric performance and demands for lead-free piezoelectric materials (in consideration of the environmental load of the lead-containing materials) are increasing, and therefore development of new lead-free perovskite-oxide materials is proceeding.
In the process of development of lead-free perovskite oxide materials, some oxides having ABO3 type composition and being theoretically considered to exhibit high piezoelectric performance are known to be actually incapable of being formed to have a perovskite crystal structure by high-temperature baking at normal pressure, but to be capable of being formed to have a perovskite crystal structure by baking at high pressure exceeding several GPa (gigapascal). For example, although the Bi-based oxides having ABO3 type composition are theoretically considered to be lead-free piezoelectric materials exhibiting high piezoelectricity (ferroelectricity), most of the Bi-based oxides having ABO3 type composition are hard or unable to be formed to have a perovskite crystal structure by high-temperature baking at normal pressure. Currently, BiFeO3 is the only Bi-based perovskite oxide which can be formed into bulk ceramic at normal pressure. On the other hand, for example, BiAlO3 can be formed into bulk ceramic having a perovskite crystal structure only by the synthesis at a temperature as high as 1000° C. and a pressure as high as 6 GPa. (See J. Zylberberg et al., “Bismuth Aluminate BiAlO3: A New Lead-free High-Tc Piezo-/ferroelectric”, Proceedings of the 16th IEEE International Symposium on Applications of Ferroelectrics, Paper No. 28PS-B13, 2007.) However, baking at high pressure requires complex equipment and an uneasy process.
Further, in order to increase the mounting density of devices, downsizing of devices is currently being pursued. In particular, development of piezoelectric devices formed with thin films is proceeding. In order to realize the thin-film piezoelectric devices, films of perovskite oxides being able to be formed by sputtering, the sol-gel technique, CVD (chemical vapor deposition), or the like and having satisfactory element characteristics are currently being studied.
In the above circumstances, some attempts to form a thin film of a material having a perovskite crystal structure by sputtering or the like have been reported, where the material has been theoretically considered able to exhibit high ferroelectricity, and has been conventionally known to be unable or hard to be formed to have a perovskite crystal structure without use of a special technique such as high-pressure synthesis. However, films of many Bi-based oxides formed by themselves at normal pressure are hard to have a perovskite crystal structure with satisfactory crystallinity, although the characteristics of the films of the Bi-based oxides depend on the compositions of the Bi-based oxides. In order to overcome this problem, an attempt to form a thin film of a Bi-based oxide so as to have a perovskite crystal structure by producing a solid solution of the Bi-based oxide and BiFeO3 has been reported, where the Bi-based oxide is known to be unable to be formed to have a perovskite crystal structure without being baked at high pressure, and BiFeO3 is known to be able to be easily formed into a thin film having a perovskite crystal structure by bulk baking at normal pressure. M. Okada et al. (in “Synthesis of Bi(FexAl1-x)O3 Thin Films by Pulsed Laser Deposition and Its Structural Characterization”, Japanese Journal of Applied Physics, Vol. 43, No. 9B, pp. 6609-6612, 2004) report that films of Bi(Fe, Al)O3 each having a perovskite crystal structure are formed on perovskite substrates of SrTiO3 by producing solid solutions of 0 to 50% BiAlO3 in BiFeO3.
The films of Bi (Fe, Al)O3 reported in the Okada reference are formed on monocrystalline perovskite substrates. Nevertheless, from the viewpoint of the cost and versatility, it is desirable that perovskite-oxide films can be formed on substrates other than the monocrystalline perovskite substrate, and it is particularly desirable that perovskite-oxide films can be formed on silicon substrates, which are especially versatile.